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Originally Posted by John Eacott
(Post 10063405)
I suspect that it hasn’t been pointed out to you because your interpretation is incorrect...
It's a scenario that gets discussed during recurrent training in the classroom, but I've never been asked to demonstrate it in the simulator. If the trainee is totally on-top of all the other malfunctions covered in the syllabus, and there is time available to look at other things, then sure, take a look at this one in the sim. |
gulli, you’re quite right and my interpretation of megan’s statement was of a No 1 engine failure. Not a No 1 drive failure.
RTFQ John!! |
With that schematic the possible failure mechanism and consequence becomes very clear! |
Originally Posted by henra
(Post 10063514)
..With that schematic the possible failure mechanism and consequence becomes very clear!
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Isn't that scenario a bit far-fetched? Surely if that shaft failed, the engine would very rapidly spin up due to the removal of the load and shut itself down?
I don't know what overspeed protection there is but if it has a DECU, it would surely shut the engine down. Has that shaft ever failed? If not, then it is quite understandable that it isn't covered in annual sim training. |
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One thing I always wondered is why on the Bell 212 tripletach the Rotor RPM is a small, short needle and the eng RPM are 2 big long needles? Isn't Rotor RPM the most important?
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Reason being, there are two N2 needles and one NR needle. If the big needle was NR, there wouldn't be enough length in the 2 shorter N2 needles so the "1" and "2" flags on the respective needles didn't overlap.
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Gullibell - I still don't see it - if that gear fails (not sure what you mean by gear failure - does it disintegrate or just stop spinning?) the gear that links it to the TR drive must surely also fail/stop spinning since it is attached to the same shaft (as far as I can see in the diagram).
And, if the gear fails and the load from the MRGB is suddenly removed leaving only the load of the TR on the No1 engine (especially in the cruise with relatively low TR thrust) there must be a Tq split until the N1 governor for No 1 engine catches up and backs off the fuel. |
Originally Posted by gulliBell
(Post 10063763)
Reason being, there are two N2 needles and one NR needle. If the big needle was NR, there wouldn't be enough length in the 2 shorter N2 needles so the "1" and "2" flags on the respective needles didn't overlap.
I was always taught...caution light, strange noise, disturbance in the force...check Rotor RPM and control that before you do or say anything else. |
yes, of course there will be a TQ split between the engines if one engine is driving the main rotor and the other is driving the TR. There will be a point in the collective range of travel where the TQ on each engine will be matched. |
Originally Posted by albatross
(Post 10063889)
Well if all three are lined up it doesn't matter...
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Lets roll back a bit here:
Nick Lappos 2nd Nov 2001, 18:09 The procedure is based on an actual failure that occurred about 15 years ago, where the input gear attachment bolts lost torque and the separation that you describe actually occurred. The gear was redesigned, and no repeat failure occurred. The flight crew noted the problem as noise and rumbling, a momentary upspeed of #1 engine, a swing to the left (extra tail thrust) and then back to normal, with very low #1 torque and high #2 torque. After a bit of discussion, the crew left well enough alone, and flew home without shutting down #1 (what a good pair of guys! If it works, leave it alone!). When they landed, they noted that the failure, in that the tail rotor was not connected to the main rotor. We id'd the problem and fixed it asap, of course, and no repeat has occurred. We inspect all boxes on overhaul for signs of lost torque on that gear to see if any recurrence is creeping back, and everything is fine now. For Nr Fairy The above is not like a more common failure that NR fairy notes, where the engine shaft going to the transmission can fail, and N2/Np can go up while torque goes down. In that case, you have an engine power loss, but a healthy rotor drive train otherwise. That can be confusing because the engine rpm on the triple tach goes up, but the rotor goes down. The rotor is your closest friend, so it is wise to make it happy first, of course. Crews can get confused when those needles, always stuck together before, start to disagree. Also, I am surprised that any A's are flogging around with disconnected electric overspeed systems. They were a pain in the early days (1979) but should be healthy now. The normal governor will catch these failures we describe, I think, so it is not essential to the failures on this thread, but the electric overspeed is helpful for internal engine failures where the power section can unbutton from the compressor, the internal engine overspeed can get very high and engine rupture is possible. We used to do touchdown autos in the old days with the RRPM obscured for the student - surprising how close you can keep it to correct as you are attuned to the noise and vibration. When DECU are involved which talk to each other to maintain constant N1 on both engines, when there are vastly different loads placed on each engine due to the MGB gear failure, then the discussion gets interesting. |
Originally Posted by RVDT
(Post 10063971)
..I think you might mean NG or N2.
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No. The DECU talk to each other to match N1, unless TQ limit on one engine is reached before the other then the DECU will stop matching N1. |
Originally Posted by RVDT
(Post 10064004)
So if you have a stronger (better performing) N1 or Ng than the other you must have Q splits all the time?
Unlike helicopters like B212 or B412, which have a TQ split limitation, there is no such limitation in a S76 fitted with DECU. As long as the engines make power assurance, flying with DECU matched N1 and a TQ split due to engines having different margins is no big deal. |
gulliBell, the 76B allowed the pilot to match either N1 or Tq. I usually matched Tq to make the dial line up in a pleasing and eye-catching manner.
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76B, who for AC, in what role, and how did it compare with regard to the rest? Have lot of time in the A and C.
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Originally Posted by Ascend Charlie
(Post 10064269)
gulliBell, the 76B allowed the pilot to match either N1 or Tq..
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Originally Posted by megan
(Post 10064298)
...Have lot of time in the A and C.
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gear fail = the shaft between the input and output end of that gear breaks, however so. However, Nick's post indicates that the problem won't happen again since the shaft and gear were redesigned - therefore not much point in teaching it in the sim. On the N1 governing by the DECU - with AC talking about matching N1 or Tq - I think that is confusing the ability to beep the engines up and down to match them in the way you prefer with what the DECU is designed to do (govern N1 and load share with the other engine) |
The B in the 76B stood for BALLS, which it had plenty of with the PT-6 and FADEC.
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Originally Posted by [email protected]
(Post 10064474)
1. so it is a shaft failure or the bolts that hold the gear to the shaft and not the gear breaking up - that makes more sense.
2. However, Nick's post indicates that the problem won't happen again since the shaft and gear were redesigned - therefore not much point in teaching it in the sim. 3. On the N1 governing by the DECU - with AC talking about matching N1 or Tq - I think that is confusing the ability to beep the engines up and down to match them in the way you prefer with what the DECU is designed to do (govern N1 and load share with the other engine) 2. As I mentioned earlier, we do not cover this malfunction in the simulator. It is discussed in class room training, more-so to get the trainees to think about and diagnose a set of circumstances which might not be covered in the RFM. Not all malfunctions are in the ECL. 3. There is no beep available to the pilot with DECU in automatic mode. The DECU gives you 107% N2 on both engines with N1 matched. That's it. The engine trim switch on the pilot collective - of which there is only one - only functions when the engine is in manual mode (i.e. using the MFCU controlling fuel flow, DECU controlled AFCU is disabled). It will beep up/down the engine/s in manual mode. Things get a bit tricky with the single eng trim switch and both engines in manual mode, particularly if there is a TQ split between the engines. This isn't usually practiced in the simulator, but it is easily manageable if you know what you're doing. |
On the N1 governing by the DECU - with AC talking about matching N1 or Tq - I think that is confusing the ability to beep the engines up and down to match them in the way you prefer with what the DECU is designed to do (govern N1 and load share with the other engine) |
Originally Posted by Ascend Charlie
(Post 10064475)
The B in the 76B stood for BALLS, which it had plenty of with the PT-6 and FADEC.
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212man - yes, my mistake, I wrote N1 when I should have put N2 - it comes from years of calling them Ng and Nf instead of N1 and N2 - doh!:ok:
Gulli - thanks, the Bell DECUs and Airbus DECUs clearly work in subtly different ways. Since many aircraft have the facility to let you beep up Nr for Cat A, either automatically or via a switch/beep, I assumed the 76 was the same. |
Originally Posted by 212man
(Post 10064634)
DECUs, and any governing system, only govern N1/Ng when the engine(s) are at idle. With the Nr in the normal operating range they govern N2/Nf.
And the other point. The DECU has no idea what the NR is...it doesn't even have an input for NR signal. The trigger for the DECU to govern N2 is when it senses the ECL in FLY position. |
Not quite. After the engine is started, the DECU only controls N1 once the N2 exceeds 9%. Which is why, if you start the engine with the rotor brake on and then move the ECL from IDLE to FLY, the engine stays at idle. And the other point. The DECU has no idea what the NR is...it doesn't even have an input for NR signal. The trigger for the DECU to govern N2 is when it senses the ECL in FLY position. I was also talking in generalities about any governor, whether a FADEC, a DECU, an ECU or a hydro-pneumatic system using P3 air and bob weights. You start the engine and the N1 is governed then, as you increase the power the N2 increases and at a certain point it becomes the datum that is then governed. The trigger for the DECU to govern N2 is when it senses the ECL in FLY position (Note - I do know that in the S92 the Nf/N2 is referred to as Np, but was keeping it simple) |
Terminology
I think we should all think carefully about..
N1/Ng N2/NP N3/Nr? Goes to show standardisation is required.. Hated Torque matching 212 - 3B engine with TCU. 412/212 with ITT trim better but can lead you away... PTGs obviously don't match perfectly and one OHC unit against a almost timex unit can be interesting. PWAC removed field cleaning from MM so orifice clean no longer an option Back to original question. Most twins have overspeed protection of some sort. Singles driveshaft failure will be interesting... Revert to the pilots... |
But Ng = gas generator rpm
Nf = free power turbine rpm and Nr = rotor rpm That is both logical and easily understood:ok: |
Nick's post indicates that the problem won't happen again since the shaft and gear were redesigned - therefore not much point in teaching it in the sim |
There was a case of a 412 in Oz, airborne for a check flight after service, and the drive shaft from the combining gearbox let go.
Two serviceable engines having a little overspeed, rotor having somewhat of an underspeed, but the cool captain pulled off a good auto and all was well. |
Originally Posted by Ascend Charlie
(Post 10065224)
..Two serviceable engines having a little overspeed, rotor having somewhat of an underspeed, but the cool captain pulled off a good auto and all was well.
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